With all due respect your shaft is toast so rather than risk damage to the tranny I would advise cutting it out. Couplings usually, and when I say this I mean like 95% of the time, are NOT reusable due to the tolerance lost from breaking the layer of rust free.
More often than not, with old shafts & couplings, you can not separate them. If you need a new shaft any way just cut it out with a 4" angle grinder and a cut off wheel. It will take three minutes max. If this is a split coupling you may get it off but if it is a solid coupling why risk it..
Also here are some warnings:
#1 Never, ever use a slide hammer to remove a prop shaft if it is connected to the tranny. I have seen inexperienced "summer help" shatter a tranny case doing this.. "Look it moved!"
#2 Never bang on the shaft or coupling if it is connected to the tranny. You can damage the bearings and create flat spots! if you must bang on it use a brass, coper or lead hammer..
#3 Always use PB Blaster, Kroil or Thrust!! Liquid Wrench, WD-40 etc. suck at penetrating!!
#4 Never, ever get any penetrating oil on or near your transmissions output shaft seal!!! It will eat it and ruin it requiring a tranny rebuild. Never use the spray feature near any output shaft seals and always dab it on with an artists brush or flux brush instead.
Your shaft is already toast, cut it before doing any potential damage to the tranny!!
Just my .02!
PSS Installation (LINK)